1. Field of Invention
The technical scope of the invention is that of methods to formulate a control order for an organ allowing a spinning projectile to be piloted in yaw and pitch, and notably an artillery projectile fired from a cannon.
2. Description of Related Art
Such methods or processes are generally implemented within an autopilot or projectile command control device and they allow the projectile's altitude and trajectory to be controlled in-flight.
In theory, projectiles fired from cannons have a well-known ballistic trajectory. However, external factors (such as wind, temperature and atmospherical pressure) or internal factors (such as initial velocity and the aerodynamic coefficients) are likely to influence the trajectory.
This results in deviations upon the projectile's impacting on the ground which, at the current maximum 155 mm artillery range (around 35 km) are close, for standard deviations, to 500 m in range and 150 m in direction.
So as to improve furing accuracy, certain solutions are known:
Thus, as described by patent FR-2786561, the projectile's range trajectory may be corrected by means of the irreversible deployment of airbrakes, controlled autonomously or from the ground. Standard deviations in accuracy, in this case, at 35 km are of around a hundred meters in range and direction.
As described in patent EP-905473, it is also possible for the trajectory to be controlled continually and autonomously by commanding the rotation (deflection) of aerodynamics fins from an autopilot device on-board the projectile.
Standard deviations in accuracy may be reduced to around ten meters, both in range and direction. Moreover, it is possible to substantially increase the maximum range (which may exceed 65 km without the need for additional propellants or modification to the weapon) by using the gliding effect.
Projectiles that are fully autonomous in flight such as those described in the second family above take their own bearings during their flight by means of a satellite positioning system (more commonly known as a Global Positioning System or GPS). Before being fired, the projectile is programmed with the target's coordinates. It determines its own position during its flight, establishers control orders using appropriate algorithms and implements the trajectory control means that allow it to reach the target defined by the coordinates.
The design constraints (resistance to acceleration) generally lead to these controlled trajectory autonomous projectiles to be given an aerodynamic architecture of the “Canard” type characterized by a rear fixed boattail and by deflectable fins located to the fore of the projectile. Such an architecture is described, for example, by patent EP-905473.
The projectile's flight stability is, in theory, ensured by the aerodynamic configuration and not by gyroscopic effect such as in a conventional shell.
However, non-reproducible aerodynamics imperfections, as well as vortexes generated by the incidence, lead to a random autorotational movement, generally fairly slow (less than 10 rps) about the roll axis. Additionally, classical control algorithms, in which the control orders in yaw and pitch only rely on index values and inertial measurements respectively of yaw and pitch, leading, for the angular roll rates that may be reached, to the selection of control motors for the deflection of the fins whose dynamic characteristics are compatible with the maximum possible rate of roll, for example a motor with a natural frequency of 100 Hz is needed for a rate of roll of 10 rps. Moreover, parasitic angular movements of the projectile, in yaw and pitch, will appear, due to the gyroscopic couplings. These, very badly damped, parasitic angular movements generate obliquities that hinder the correct ballistic behavior of the projectile and result in a reduction of the maximum range of the projectile by a few kilometers.
One solution used on certain missiles, allowing these couplings to be suppressed, consists in piloting the projectile in roll, that is to say, by applying commands allowing the parasitic aerodynamic roll moments to be offset and either the angular rate of roll to be eliminated, or the roll angle to be kept constant. However, it is known that a “Canard” type finned projectile can only be piloted in roll on condition that the rotation of the stabilizing boattail be uncoupled (by being mounted on bearings, for example) so as to eliminate the aerodynamic moments of interaction that are applied to the boattail and which oppose the roll moments created by the differential deflections of the Canard fins. Such a mechanical uncoupling device, for a projectile fired from a cannon, proves particularly fragile and costly. Additionally, the addition of a piloting roll loop results in an increase in the “flight control” function cost.